22
R. Ebinghaus et al.
Results from the geological heat flux approach suggest that mercury transfer
through mid-ocean ridges could be on the order of 1900 to 3800 t year-I, while
the whole oceanic crust would emit 7300 to 14 700 t year- I Hg (Rasmussen 1994).
These results, in combination with other sources of submarine mercury like
hydrothermal vents, seismic activity, and erosion of ocean ridges, as well as
submerged parts of the continents, could give the impression that atmospheric
deposition might be negligible in the total oceanic mercury budget (Camargo
1993). However, other authors assume that deposition is the major source of
mercury (re)emitted to the atmosphere from ocean surfaces (Fitzgerald 1993).
These findings contradict each other to such a degree that two main questions
have to be asked: (1) is crustal degassing really the major source of oceanic Hg
and (2) if so, then what percentage of that Hg actually reaches the atmosphere?
These pathways (and their terrestrial counterparts) definitely have to be
investigated much more thoroughly (if possible, by actual flux measurements)
before the large discrepancies in global Hg budgets can be resolved. It should,
however, be noted that the scientific community is beginning to develop
programs for expansion of direct measurement campaigns in international
collaborative studies (e.g., Gustin and Lindberg 1997)
4.4
Natural Air/Surface Exchange Processes with Soils, Oceans, Freshwaters,
and Vegetation
This section will attempt to generalize the common features of mercury exchange
between the atmosphere and terrestrial or aquatic compartments and general
parameters determining direction and magnitude of the observed fluxes. It has to
be kept in mind, though, that only gaseous dry deposition and volatilization are
directly comparable in this way. The net emission that is usually observed as a
sum of these two processes has to be balanced against particulate dry deposition
(generally small) and wet deposition (generally important) in any study to finally
evaluate a compartment as a source or a sink of atmospheric mercury.
The exchange of gaseous mercury at any interface is driven by a concentration
gradient. If one compartment has a higher Hg(o) concentration than another, a
net emission will take place, while net deposition will occur if the competing
compartment is clean compared to the overlying atmosphere. Since many
investigated waterbodies seem to be supersaturated with gaseous Hg compounds
compared to the atmosphere, mostly evasion fluxes from waters to the air have
been reported. However, under some conditions (at night and following periods
of very high winds) downward fluxes to waters in subtropical Florida have been
measured (Lindberg et al. 1997). This is also mostly true for soils, but there are
some measurements over background soils suggesting net deposition under
limited conditions, also mostly at night or over very wet soils (Kim et al. 1995). In
earlier work, it has been shown that soils adsorb Hg vapor if exposed to elevated
concentrations (Klusman and Matoske 1983). Therefore, it seems appropriate to
propose the existence of a "compensation point", meaning that soils below a
certain Hg(o) concentration (or probably: soil gas concentration) tend to absorb
R. Ebinghaus et al.
Results from the geological heat flux approach suggest that mercury transfer
through mid-ocean ridges could be on the order of 1900 to 3800 t year-I, while
the whole oceanic crust would emit 7300 to 14 700 t year- I Hg (Rasmussen 1994).
These results, in combination with other sources of submarine mercury like
hydrothermal vents, seismic activity, and erosion of ocean ridges, as well as
submerged parts of the continents, could give the impression that atmospheric
deposition might be negligible in the total oceanic mercury budget (Camargo
1993). However, other authors assume that deposition is the major source of
mercury (re)emitted to the atmosphere from ocean surfaces (Fitzgerald 1993).
These findings contradict each other to such a degree that two main questions
have to be asked: (1) is crustal degassing really the major source of oceanic Hg
and (2) if so, then what percentage of that Hg actually reaches the atmosphere?
These pathways (and their terrestrial counterparts) definitely have to be
investigated much more thoroughly (if possible, by actual flux measurements)
before the large discrepancies in global Hg budgets can be resolved. It should,
however, be noted that the scientific community is beginning to develop
programs for expansion of direct measurement campaigns in international
collaborative studies (e.g., Gustin and Lindberg 1997)
4.4
Natural Air/Surface Exchange Processes with Soils, Oceans, Freshwaters,
and Vegetation
This section will attempt to generalize the common features of mercury exchange
between the atmosphere and terrestrial or aquatic compartments and general
parameters determining direction and magnitude of the observed fluxes. It has to
be kept in mind, though, that only gaseous dry deposition and volatilization are
directly comparable in this way. The net emission that is usually observed as a
sum of these two processes has to be balanced against particulate dry deposition
(generally small) and wet deposition (generally important) in any study to finally
evaluate a compartment as a source or a sink of atmospheric mercury.
The exchange of gaseous mercury at any interface is driven by a concentration
gradient. If one compartment has a higher Hg(o) concentration than another, a
net emission will take place, while net deposition will occur if the competing
compartment is clean compared to the overlying atmosphere. Since many
investigated waterbodies seem to be supersaturated with gaseous Hg compounds
compared to the atmosphere, mostly evasion fluxes from waters to the air have
been reported. However, under some conditions (at night and following periods
of very high winds) downward fluxes to waters in subtropical Florida have been
measured (Lindberg et al. 1997). This is also mostly true for soils, but there are
some measurements over background soils suggesting net deposition under
limited conditions, also mostly at night or over very wet soils (Kim et al. 1995). In
earlier work, it has been shown that soils adsorb Hg vapor if exposed to elevated
concentrations (Klusman and Matoske 1983). Therefore, it seems appropriate to
propose the existence of a "compensation point", meaning that soils below a
certain Hg(o) concentration (or probably: soil gas concentration) tend to absorb